This invention pertains to making viscous clutch assemblies. More particularly, this invention pertains to the design of robust and more readily manufacturable, continuously controllable, magnetorheological fluid fan drive assemblies.
U.S. Pat. Nos. 5,960,918; 6,032,772; 6,102,177; and 6,173,823, each entitled xe2x80x9cViscous Clutch Assembly,xe2x80x9d describe clutches for a vehicle cooling fan assembly that use a magnetorheological (MR) fluid as the viscous material operating in a gap between the engine driven rotor and the fan carrying stator. The assembly further includes a coil for creating an electromagnetic field in the gap to vary the yield stress of the MR fluid and, thus, the speed of the fan.
U.S. Pat. No. 5,667,715 entitled xe2x80x9cMagnetorheological Fluidxe2x80x9d and U.S. Pat. No. 6,149,832 entitled xe2x80x9cStabilized Magnetorheological Fluid Compositionsxe2x80x9d describe viscous fluids suitable for use in the viscous clutch assemblies. Often these fluids comprise suitable, finely divided iron particles suspended in a nonpolar vehicle. The fluids are formulated to resist particle separation even under high separation force applications and they typically function as Bingham fluids. In an ambient gravitational field, but in the absence of a magnetic field, they display a shear stress that increases generally linearly as the shear rate on the fluid is increased. When magnetorheological Bingham fluids are subjected to a magnetic field, the shear stress vs. shear rate relationship is increased so that substantially more shear stress is required to commence shear of the fluid. This characteristic is very useful in controlling the transfer of torque between a rotor and stator in a viscous fluid clutch assembly.
In engine driven fan drive systems of the type described the speed of the fan is continuously variable by varying the magnetic flux in the MR fluid. Such variable speed fan drive assemblies provide vehicle fuel economy improvement, noise reduction, powertrain cooling improvement and cost reduction. After evaluation and testing of fan drive assemblies such as those described in the above four patents for a specific truck application it is realized that further improvements could be made. It is an object of this invention to provide improvements in the design of certain fan drives for the purpose of their ease of manufacture and robustness of operation.
As described in the xe2x80x9cViscous Clutch Assemblyxe2x80x9d patents identified above, a fan drive assembly has an engine driven input shaft with an attached hub and rotor assembly. This input assembly applies torque to a fan drive assembly using a viscous fluid, preferably a magnetorheological fluid such as those described in the above cited patents. Accordingly, the input and output structures are designed with complementary rotating portions that fit closely together with a thin layer of torque transmitting, MR fluid between them.
Also positioned close to this fluid gap is an electric coil for generating a variable magnetic field in the fluid to vary its yield stress and, thus, the torque transmitted from the input shaft/rotor assembly to the fan drive. A separate computer based controller determines the voltage or current flow applied to the coil. Experience with such fan drive mechanisms reveals the advantage of careful design of the complementary fluid gap forming portions of the input and fan drive assemblies and the means taken to seal in the MR fluid. This invention provides several such related improvements enhancing the ease with which the fan drive is made and the robustness of the drive.
In accordance with a preferred embodiment of the invention, a viscous fluid clutch for a vehicle cooling fan drive comprises a driving shaft/rotor assembly enclosed by a fan housing and a fan cover assembly. The fan housing is carried on the driving torque input shaft but separated from the shaft with respect to torque transmission by a bearing. A fan cover assembly that is attached to the fan housing includes a clutch stator insert that receives the rotor and the MR fluid for the transmission of torque.
The fan cover assembly includes a fan cover body, a fan cover insert and an annular coil body with coil windings. The coil body is carried on the circular, ferrous metal fan cover insert that, in turn, is preferably cast in place within the fan cover body. This assembly is co-axial with the input shaft.
The fan cover insert has a larger diameter than the coil and the outer region of the insert contains a slot in which the rotor is received during assembly of the drive. The slot and the rotor ring leave gaps on both sides of the rotor for the MR fluid. And the magnetic permeability of the ferrous composition on both sides of the slot confines the magnetic field of the coil on the MR fluid in the gaps.
One important feature of the invention is the method by which the fan cover assembly is made. The fan cover insert is made as a single round, disc-like precursor piece, preferably by hot forging a steel billet. The fan cover body is cast around the hot forged insert precursor using a suitable aluminum alloy. Anchoring features are formed on the fan cover insert to prevent separation from the cover body. Further processing of this composite part includes machining a circular slot through the insert for the rotor. This operation divides the round insert into two parts, both of which are anchored in the aluminum body portion of the composite. The separated portions of the insert define a slot for the rotor and MR fluid and provide magnetically permeable regions to concentrate the magnetic field in the fluid.
Additional machining of the fan cover insert provides a circular channel to receive the coil body and to provide a passage for the coil leads to a non-rotating assembly for supplying power to the coil. Additional machining of the fan cover body provides for improved sealing engagement with the fan housing member.
Other objects and advantages of the invention will become more apparent from a detailed description of preferred embodiments which follows.